Advances with charged particle beam transfer devices as used for making integrated circuits and related devices have made it possible to achieve both improved transfer pattern resolution and improved throughput (productivity) in recent years. One such device that has been investigated in the past uses a one-shot transfer system that transfers one die (meaning an entire pattern for one of multiple integrated circuits formed on a single wafer) or multiple dies' worth of patterns from a mask onto a sensitive substrate in a single exposure ("shot"). However, with one-shot transfer systems, the masks which comprise the transfer original are difficult to make, and it is difficult to keep aberrations in the charged particle optical system (hereinafter referred to simply as the "optical system") to below a desired value inside of a large optical field of one die or greater. Therefore, devices using divided transfer systems have recently been studied which divide the pattern which is to be transferred onto the sensitive substrate into multiple subfields, each subfield being smaller than the size of one die.
FIG. 13 shows an example of an electron beam reducing transfer device using a conventional divided transfer system. An electron beam EB propagating from an electron beam source (not shown), having a cross section having a square shape, is deflected a set distance .delta. from the optical axis AX of the optical system by a deflecting system (not shown) and guided to one of the multiple mask subfields 2a situated in a mask 2. The mask subfield 2a is an area that includes an electron-beam transmitting area corresponding with the pattern shape that is to be transferred onto the wafer 5. Each of these mask subfields 2a is separated from one another by a boundary field 2b that either blocks or scatters the electron beam.
The electron beam EB, having passed through the mask subfield 2a, passes through a first projection lens (not shown) and converges once at a crossover (electron beam source image) CO, after which the beam passes through a second projection lens (not shown). The beam is focused on a single transfer subfield 5b on the wafer 5 to which an electron beam resist has been applied, as the sensitive substrate. Thus, the image of the pattern in the mask subfield 2a is projected at a desired reduction ratio (e.g., 1/4) onto the transfer subfield 5b. In FIG. 13, the Z axis is drawn parallel with the optical axis AX, and the X axis and Y axis are drawn parallel to the two orthogonal edges of the mask subfield 2a, inside the plane that is perpendicular to the Z axis.
During transfer of the entire pattern on the mask 2, irradiation of the electron beam EB is repeated using the mask subfield 2a as a unit, and reduced images of the patterns on the mask subfields 2a are sequentially transferred to separate transfer subfields 5b on the wafer 5. The position of each transfer subfield 5b on the wafer 5 is determined by a beam-deflecting system (not shown) which is situated in the optical path between the mask 2 and the wafer 5 so that the transfer subfields 5b, individually corresponding with a separate respective mask subfield 2a, touch each other. The electron beam EB that has passed through the mask subfield 2a is focused onto the wafer 5 by means of a first projection lens and a second projection lens (not shown). The image of not only the mask subfield 2a is projected, but also that of the boundary field 2b, which is transferred at a desired reduction ratio, creating an unexposed area corresponding with the boundary field 2b between each transfer subfield 5b. Therefore, the transfer positions of the pattern images are shifted only by a width corresponding to the boundary field 2b.
Once the pattern images corresponding with all of the mask subfields 2a of a mask 2 have been transferred, the transfer of patterns comprising one die's worth 5a of transfer subfields 5b on the wafer 5 is completed. During this time, transfer is performed while correcting any aberrations, etc., such as the focal position of the subfield image which is formed on the transfer reception field for each subfield, or field distortion, etc. Thus, transfers of excellent resolution and accuracy can be accomplished across an optically wider field than with one-shot transfer systems. Japan Kokai Patent Application Publication No. HEI 5-36593 discloses a method of forming a mask subfield that corresponds with a repetitious pattern, such as memory cells, and then repeatedly transferring the image of this subfield to multiple locations on a wafer. Such a scheme uses a variable shaping aperture, without independently producing patterns other than the repetitious pattern on the mask. Unfortunately, throughput is very low with such a scheme.
In the divided transfer method described above in connection with FIG. 13, there is a 1:1 correlation between the mask subfields 2a on the mask 2 and the transfer subfields 5b on the wafer 5. For example, when one die's worth 5a consists of 100 transfer subfields 5b, there are 100 corresponding mask subfields 2a in the mask 2. In order to reductively transfer the patterns from the mask 2 to the wafer 5, the size of a single mask subfield 2a is the size of a single corresponding transfer subfield 5b magnified by the reciprocal of the reduction ratio. In addition, boundary fields 2b are also situated in the mask along with the mask subfields 2a. Consequently, the size of the range 2P in which the mask subfields 2a are situated on the mask (hereinafter referred to as the "pattern field") is always larger than the total surface area of the subfields on the mask 2 corresponding to all of the transfer subfields 5b on the wafer. In the present explanation (including FIG. 13), the pattern field 2P and the transfer subfields 5b are seen as corresponding with one die's worth. Ordinarily, .delta. cannot be longer than one side of one die because otherwise large optical aberrations would be introduced. Therefore, one die is divided into multiple "stripes," and the position of each stripe is determined by movement of the stage.
In an electron beam transfer device, the more the electron beam irradiation position is separated from the optical axis AX of the optical system, the greater the possible aberration imparted by the optical system. In other words, the greater the deflection amount .delta. shown in FIG. 13, the greater optical errors in resolution, etc., will become. When considering the range in which optical errors are kept within the tolerance range on the mask side and on the wafer side, both ranges are considered to be circular areas centered around the optical axis AX of the optical system on both the mask surface and the wafer surface. Herein, the range in which optical errors are kept within the tolerance range on the mask side will be referred to as the "mask-side optical field" Cm, and the range in which optical errors are kept within the tolerance range on the wafer side will be referred to as the "sensitive substrate-side optical field" Cw.
In the case of reductive transfer, because the mask-side optical field Cm is considerably larger than the sensitive substrate-side optical field Cw, the size of the pattern field 2P on the mask 2 and of the corresponding transfer field 5a on the wafer 5 are limited by the size of the mask-side optical field Cm. In other words, even if there is surplus optical field on the wafer side, the pattern can be transferred to no more than a limited smaller range. Consequently, the optical field on the wafer should be set small, and the number of stripes increased. Such a scheme decreases throughput. As a result, the percentage of the total process time taken up by stage driving time to move the wafer 5 increases, thereby decreasing throughput. Throughput is greatly decreased especially during stage return (when changing drive directions), when nonproductive time increases due to the accumulation of overhead time.
Therefore, a system has been considered to prevent this kind of decrease in throughput in which, in several of the mask subfields, multiple subfields with the same pattern are arranged in a single subfield and the pattern in the single subfield is repeatedly transferred to multiple transfer subfields on the wafer. The kind of mask in which common patterns are thus arranged is called a "compressed mask."
Especially when transferring patterns having many common circuit patterns, as with, e.g., dynamic random access memory (DRAM), the mask can be made smaller by one or more orders by performing transfer by a divided transfer system using a compressed mask. This eliminates the need to move the mask at high speeds during transferring. However, if the arrangement of the pattern and the transfer sequence of the patterns are not considered when using a compressed mask, the amount of movement by the deflecting system (deflection movement amount) may become excessively great when, e.g., moving from one mask subfield to the next mask subfield. Problems arise as the amount of deflection movement increases between subfields when this kind of continuous transfer is being performed because longer settling time is required for the deflection system, which results in decreased throughput.
When a compressed mask is used, the pattern in a single mask subfield is repeatedly transferred, over and over, to many different transfer subfields on the wafer. However, the repeatedly transferred mask subfield is irradiated by the charged particle beam many times in succession, which increases the temperature of that subfield and causes decreased positional accuracy of the pattern within that subfield due to the resulting thermal expansion of the mask substrate.
Consideration has been given to controlling such temperature increases resulting from repeated charged particle beam irradiation of a single mask subfield. Multiple adjacent subfields with the same pattern can be formed, and these adjacent multiple subfield patterns are sequentially irradiated over multiple cycles. However, there are problems with this cyclical irradiation of multiple subfields in that it invites decreases in throughput due to increased deflecting-system settling time compared to using a single subfield. This is particularly true when an electromagnetic deflecting system is used, which requires long settling times, further lengthening the overall settling time. (Electromagnetic deflecting systems are normally used rather than electrostatic deflecting systems in order to decrease aberration when performing large deflection. The use of an electrostatic deflecting system for all of the mask subfields presents aberration problems.)
In addition, in order to easily manufacture masks, it is advantageous if the boundary fields 2b that separate the mask subfields 2a form a regular grid. Furthermore, since the charged particle beam irradiation area of the various mask subfields is constant, when the size of the mask subfields changes within a single mask, it becomes impossible to irradiate the entire surface of the pattern inside each mask subfield, and there is the danger that the adjacent subfield will be simultaneously irradiated. In addition, there are cases in which it is difficult, e.g., to distribute the originals of multiple various patterns with different repetition pitches into various different mask subfields of the same size.
Furthermore, when transferring a pattern using a charged particle beam such as an electron beam or ion beam, so-called Coulomb effect blurring occurs due to the repulsion between the charged particles, causing a blurring phenomenon in the transferred image. This effect limits the current density that can be used during transfer. Therefore, the current density during transfer to the wafer must be limited using, as a standard, the transfer subfield on the wafer that received the greatest amount of irradiation. When this is done, there are many subfields in which a surplus of electrical current could be used but is not used, presenting the problems of lengthened irradiation time and an inability to increase throughput.
This invention addresses these problems, and has as its first object to provide a charged particle beam transfer method which is able to shorten the spaces between subfields which are continuously transferred on a mask when performing transfer by a divided transfer system using a compressed mask, thereby improving throughput.
A second object of this invention is to provide a charged particle beam transfer method which, when performing transfer by a divided transfer system using a compressed mask, prevents irradiation of the charged particle beam for long periods of time on a specified mask subfield, thereby enabling the positional accuracy of the pattern being transferred to be maintained at a high level.
A third object of this invention is to provide a charged particle beam transfer method which, when performing transfer by a divided transfer system, is able to perform deflection of the charged particle beam at high speeds during cyclical transferring of the patterns in multiple adjacent mask subfields, and is able to perform deflection of the charged particle beam between separated subfields with a high level of accuracy.
A fourth object of this invention is to provide a charged particle beam transfer method that, when performing pattern transfer by a divided transfer system using a compressed mask, performs compressing the pattern to fewer mask subfields and projecting them, even for multiple types of cyclical patterns with different repetition pitches on the sensitive substrate.
A fifth object of this invention is to provide a charged particle beam transfer method that, when performing pattern transfer by a divided transfer system, eliminates mask subfields in which there is a particularly large amount of irradiation from the charged particle beam, hence making it possible to prevent blurring due to the Coulomb effect without excessively decreasing throughput.
These and other objects of the invention will be better understood by reference to the following summary and detailed description of the invention.